The present invention relates to a device as well as a process for smoothing printing screens for silk screen printing, in particular rotary screens.
Printing screens for silk screen printing consist of nickel foil, which is so thin that it can be easily creased during the process or by the operator""s carelessness. Crease marks however disrupt the uniform pattern application, which is why they have to be removed before the printing screen for silk screen printing can be re-used.
The device according to the invention achieves this object by the fact that a compression-resistant and dimensionally stable support preferably developed as a rotatable support roller for the screen as well as a nozzle unit for spraying a jet of liquid onto the screen at high pressure are provided, the screen lying on the support at least in the working area of the nozzle unit.
The process according to the invention provides for a liquid to be sprayed at high pressure onto a screen lying on a compression-resistant and dimensionally stable support.
The device according to the invention as well as the associated process are based on devices which are used for the removal of a varnish deposited for patterning purposes on the outside of the printing screen for silk screen printing. For this process, called xe2x80x9cstrippingxe2x80x9d, it is known to spray water at high pressure onto the printing screen for silk screen printing. A corresponding device is shown for example in DE 42 24 511 A1. For the known device, the support roller is formed by an inflatable supporting tube which is enclosed by a thin metal foil. The result of this is that the support roller is deformed under the pressure of the impacting jet of liquid which is why, for the known device, crease marks additional to the already present creases occur during the xe2x80x9cstrippingxe2x80x9d in most cases.
On the other hand, if the support is formed from compression-resistant and dimensionally stable material, then a hard and unyielding abutment is obtained on which the placed screen is smoothed by the action of the jet of liquid. The development of the support as a support roller is particularly advantageous in design terms.
The smoothing effect occurs in particular at pressures above 1000 bar, a pressure of approx 2000 bar preferably being used.
A support or support roller made from high-grade steel has proved to be a suitably hard abutment. Other hard material can also be used however, such as for example a roller treated with hard chromium.
An optimal smoothing effect is achieved if the distance between the nozzle tip of the nozzle unit and the screen laid on the support roller is between 5 mm and 20 mm, preferably between 8 mm and 10 mm.
In order to be able to use the device according to the invention for any screen size, it is favourable if the device has a free space beneath the support roller for a screen section suspended freely beneath the support roller. Using the free space beneath the support roller, it is possible for the support roller to be relatively small, so that, even for the smallest rotary screen size, only a part of the screen lies on the support roller, whereas a freely suspended screen section finds space beneath the support roller. Thus, a uniform support roller can be used for all rotary screen sizes.
Not only is a smoothing of the screen achieved by the device according to the invention but also a removal of the varnish deposited for patterning purposes, as is already known. In this connection it is favourable if a rinsing device is provided beneath the support roller for the freely suspended screen section. The function of the rinsing device is to remove separated colour particles from the freely suspended screen section.
In order to ensure a full fit of the screen in the working area of the nozzle unit, a pressure roller for pressing the screen against the support roller can be preferably provided. For adaptation to different screen widths, it is advantageous if the width of the pressure roller can be changed by means of additional elements.
It is favourable if the support roller has areas, with slightly increased diameter, which form stops. The two stops form a lateral limitation for the screen pushed onto the support roller, whereby a lateral xe2x80x9cdownward migrationxe2x80x9d of the screen on the side is avoided during the course of the rotary movement of the support roller. For adaptation to different screen widths, it is advantageous if at least one of the areas is designed to be displaceable, preferably as a displaceable sleeve.
Furthermore it can be provided that the support roller is clamped or housed on one side. The cantilevered arrangement of the support roller makes it possible to push the rotary screens from the free side in axial direction onto the support roller.